In the operation of liquid sodium or sodium-potassium metal cooled nuclear reactors for power generation, it may be necessary to shut down the fission reaction of the fuel to deal with emergencies or carry out maintenance services. Reactor shut down is attained by inserting neutron absorbing control rods into the core of fissionable fuel to deprive the fuel of the needed fission producing neutrons. However, decay of the fuel in the shut down reactor continues to produce heat in significant amounts which must be dissipated from the reactor unit.
The heat capacity of the liquid metal coolant and adjacent structure aid in dissipating the residual heat. However, the structural materials of the nuclear reactor may not be capable of safely withstanding prolonged high temperatures. For example the concrete of the walls of the typical housing silo may splay and crack when subjected to high temperatures. Accordingly, auxiliary cooling systems are commonly utilized to safely remove heat from the nuclear reactor structure during shut down.
Conventional nuclear reactors have utilized a variety of elaborate energy driven cooling systems to dissipate heat from the reactor. In many of the situations warranting a shutdown, the energy supply to the cooling systems make the cooling systems themselves subject to failure. For example, pumps and ventilation systems to cool the core may fail. Furthermore, if operator intervention is necessary, there are foreseeable scenarios in which the operator would be unable to provide the appropriate action. The most reliable and desirable cooling system would be a completely passive system which could continuously remove the residual heat generated after shutdown.
Liquid metal cooled reactors such as the modular type disclosed in U.S. Pat. No. 4,508,677, utilizing sodium or sodium-potassium as the coolant, provide numerous advantages. Water cooled reactors operate at or near the boiling point of water. Any significant rise in temperature results in the generation of steam and increased pressure. By contrast, sodium or sodium-potassium has an extremely high boiling point, approximately 1800 degrees Fahrenheit at one atmosphere pressure. The normal operating temperature of the reactor is about 900 degrees Fahrenheit. Because of the high boiling point of the liquid metal, the pressure problems associated with water cooled reactors and the steam generated therein are eliminated. The heat capacity of the liquid metal coolant permits the coolant be heated several hundred degrees Fahrenheit above normal operating temperatures without danger of structural failure in the reactor.
The reactor vessels for pool-type liquid-metal cooled reactors are essentially open top cylindrical tanks without any perforations to interrupt the integrity of the vessel walls. Sealing of side and bottom walls is essential to prevent the leakage of liquid metal from the primary vessel. The vessel surfaces must also be accessible for the rigorous inspections required by safety considerations.
In the typical sodium cooled reactor, two levels of sodium circuit loops are used. Usually, a single primary circuit loop and two or more secondary circuit loops are used. The primary circuit loop contains very radioactive sodium which is heated by the fissionable fuel rods. The primary circuit loop passes through heat exchangers to exchange the heat from the fuel with one of the nonradioactive secondary sodium circuit loops. In general, sodium cooled reactors are designed to incorporate redundant secondary circuit loops in the event of failure of one circuit loop.
Upon shutdown of the reactor by fully inserting the control rods into the core of fuel, residual heat continues to be produced and dissipated according to the heat capacity of the plant. Assuming that the reactor has been at full power for a long period of time, during the first hour following shutdown an average of about 2% of full power continues to be generated. The residual heat produced continues to decay with time.
This invention comprises an improvement in the passive cooling system for removing shutdown decay heat from a liquid metal cooled nuclear reactor disclosed and claimed in U.S. Pat. No. 4,678,626, issued Dec. 2, 1985.
The disclosed contents of the above noted U.S. Pat. Nos. 4,508,677 and 4,678,626, comprising related background art, are incorporated herein by reference.